(1) Field of the Invention
The invention relates to an accessory gearbox and more specifically to an accessory gearbox integrated with a stationary frame of a gas turbine engine.
(2) Description of the Related Art
In an axial-flow gas turbine engine, incoming air is directed rearward by a large diameter fan, where the air is typically split into two discrete streams: a core air stream and a bypass air stream. The core air stream enters the engine and is pressurized in series by one or more rotating compressors and is then mixed with fuel in a central combustor. The fuel and air mixture is burned in and expelled from the combustor as hot combustion gases. The hot combustion gases are directed rearward to one or more turbines disposed downstream of the combustor. The turbines extract power from the combustion gases and use the power to rotate the compressors via a common rotor shaft or rotor spool. Conversely, the bypass air stream is directed outside the engine through one or more bypass ducts. The exhausting combustion gases in combination with the bypass air stream generate a propulsive thrust for powering a vehicle such as an aircraft.
Besides producing thrust, a gas turbine engine also supplies power for various systems and accessories essential to the engine and aircraft operation as well as passenger comfort. These systems and accessories are typically powered pneumatically by bleeding core air from the compressors or powered mechanically by engaging the driving means. Since the driving means are located proximate a central longitudinal axis of the engine, a transfer shaft is necessary to transmit the power to an accessory gearbox mounted in an external location. The accessory gearbox allows each accessory to run at an optimum speed and in an accessible location for ease of maintenance, reduced weight and minimum complexity. Accessory gearboxes are conventionally mounted beneath the engine, proximate the combustor, or beneath a cylindrical case surrounding the fan.
Accessory gearboxes mounted beneath the engine require an angle gearbox and two separate transfer shafts to transmit power from the driving means. The first transfer shaft, sometimes referred to as a tower shaft, transmits the power from the driving means, radially downward to the angle gearbox. The second transfer shaft, sometimes referred to as a lay shaft, transmits the power axially rearward from the angle gearbox to the accessory gearbox. The requirement for an angle gearbox housing, lay shaft and accessory gearbox housing adds substantial weight and complexity to the engine. Accessory gearboxes mounted beneath the engine also contribute to engine flexure, which is known to negatively affect compressor and turbine clearances and thus reduces engine fuel efficiency. Also, the area beneath the engine houses many pipes, wires and brackets, which must be tightly packaged around the accessory gearbox. The inaccessibility of the gearbox causes difficulties for maintenance personnel during diagnostic and repair procedures.
Accessory gearboxes mounted beneath the fan case require an extended tower shaft because the fan is very large in diameter. A mid-span bearing and an angle gearbox are used to transmit power from the driving means. The tower shaft transmits the power from the driving means, substantially radially downward and through the fan case to the angle gearbox. The power is transmitted axially forward from the angle gearbox to the accessory gearbox. The requirement for an angle gearbox housing and extended tower shaft with a mid-span bearing adds substantial weight and complexity to the engine. Also, accessory gearboxes mounted beneath the fan case increase the frontal area of the engine, increasing the aircraft drag and thus increasing the engine fuel burn.
What is therefore needed is an accessory gearbox that contributes minimally to engine weight and complexity, while not negatively impacting engine fuel efficiency.